Typically, the treating liquid for the paper web is applied through an opening in a container which is slit-shaped and extends transverse to the direction of movement of the paper web. The web is passed over the slit opening through which the treating liquid is applied without return flow. An illustration of such an arrangement is shown in U.S. Pat. No. 3,941,902.
In order to obtain a coat of treating liquid which is constant per surface unit, the flow/web speed ratio can be maintained constant. This can be effected, for example, by employing a gear pump to pump treating liquid, the operation of the gear pump being coupled to the movement of the paper web, so that a change of the web speed causes the rate of flow to be adjusted accordingly. During stable operation, the flow, as well as the web speed, preferably is held constant.
However, it is impossible to obtain a coat which completely covers the paper web with a small rate of flow. In other words, to obtain a complete covering, a minimum flow or a minimum coat is required, below which minimum the web is only partially covered.
It was found that this minimum coat depends on several different variables, such as the configuration of the slit, web speed, viscosity of the treating liquid and surface roughness. In a particular case, first-mentioned variables are typically constant while the latter one, i.e., the surface roughness of the paper, can vary due to the fact that the properties of a paper web seldom are constant along the web. As it is desired to obtain a complete coverage of the entire web, it is necessary to adjust the flow/web speed ratio so that it also covers the highest surface roughness. To accomplish this, the coat is usually given an unnecessary thick coating in the remaining portions of the web, which is a substantial disadvantage, as it is inefficient and wasteful.
It was found that the liquid pressure in the slit opening varies with the degree of coating. (See FIG. 1.) At coating degrees below 100 percent, i.e., when the paper web is not covered completely, the pressure rises slowly as the degree of coating increases, whereas the pressure rises at an increased rate when the coating degree increases above 100 percent. This implies that a coating degree directly above 100 percent can be permanently ensured by maintaining the pressure in the slit constant. However, this cannot be achieved, since the pressure in the slit varies with the surface structure of the web; also, the coat of treating liquid varies with the surface structure, i.e., the amount of active substance applied varies along the paper web. Consequently, as the surface properties of the paper web are non-uniform, the pressure in the slit does not remain constant.
Broadly, it is an object of the present invention to provide a method which overcomes one or more of the aforesaid problems. Specifically, it is within the contemplation of the present invention to provide a method of surface treating paper webs which gives a uniformly thick coating of the active substance in all portions of the paper web, even in those sections having the highest surface roughness.
It is a further object of the present invention to provide a method which varies the amount of coating material applied to the paper web in response to the changing surface properties of the moving paper web.